src/install: do not print image filename for install hook
src/install: do not print image filename for install hook

When having an install hook, the image file does not necessarily dictate
the update (and may also fully be left out). Thus instead of printing

| rauc-Message: 02:07:21.453: Updating /tmp/appdev with image.ext4

we print

| rauc-Message: 02:07:21.453: Updating /tmp/appdev with 'install' slot hook

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,jluebbe/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc